1. Field of the Invention
The present invention relates to a process for the production of elemental sulfur from sulfur dioxide. In one of its more particular aspects, this invention relates to the reduction of sulfur dioxide to elemental sulfur by a carbonaceous material in the presence of a molten salt. In another aspect this invention relates to the simultaneous reduction of sulfur dioxide to elemental sulfur and an alkali metal sulfate to the corresponding alkali metal sulfide. The simultaneous reduction of the sulfur dioxide and sulfate is desirable in flue gas desulfurization processes wherein some sulfate is unavoidably produced along with the sulfite-bisulfite compounds from which the sulfur dioxide is regenerated.
2. Prior Art
The need to eliminate sulfur-containing gases from flue gases has resulted in many different methods being suggested for this purpose. In some of these methods the flue gases are desulfurized without recovering any of the sulfur values in usable form. In other words, processes have been provided to recover the sulfur values as either hydrogen sulfide or elemental sulfur. In those processes recovering the sulfur values as hydrogen sulfide, the hydrogen sulfide thereby produced is generally utilized either in a sulfuric acid producing plant or a Claus plant for the production of sulfur.
Thus the conversion of sulfur-containing compounds involved in flue gas desulfurization to elemental sulfur has achieved some importance as the need for environmental clean-up has grown.
U.S. Pat. No. 3,438,733 describes a process for producing sulfur by reducing molten alkali metal sulfite with carbon. The alkali metal sulfite is formed by absorption of sulfur dioxide using a molten alkali metal carbonate as an absorbent. The process is conducted by trickling the molten alkali metal sulfite in admixture with molten alkali metal carbonate over a solid carbonaceous bed, with the sulfur thereby produced being recovered in the form of gaseous elemental sulfur. Although elemental sulfur is produced in this process, it is necessary to operate the reactor at relatively high pressures in order to convert the sulfite to elemental sulfur and to minimize the formation of alkali metal sulfide.
U.S. Pat. No. 3,904,387 discloses a process in which sulfur dioxide and carbon are heated together to produce a gaseous mixture of carbon monoxide and elemental sulfur. The process is carried out in a gasifier into which hot carbonaceous matter and a heated stream of concentrated sulfur dioxide are introduced. This process thus requires extensive heating of all carbonaceous matter and the sulfur dioxide.
U.S. Pat. No. 4,095,953 describes a modular system for reducing sulfur dioxide which consists of a plurality of compartments having coal inlets associated with each compartment and a single source of sulfur dioxide. In this modular system it is required that burners be situated in each compartment in order to heat the reactants to the proper reaction temperature, namely one between 1100.degree. and 1550.degree. F. (about 595.degree. to 845.degree. C.).
Other processes in which sulfur is formed directly from sulfur dioxide utilize hydrogen sulfide as a reactant.
For example, in U.S. Pat. No. 3,170,766 there is disclosed a process which involves the reaction of hydrogen sulfide with sulfur dioxide in an organic solvent to produce sulfur as the product of the reaction.
U.S. Pat. No. 3,441,379 discloses a similar process in which the reaction of hydrogen sulfide and sulfur dioxide is carried out in a liquid phosphoric acid ester as the reaction medium.
In U.S. Pat. No. 3,447,903 the reaction of hydrogen sulfide and sulfur dioxide is carried out in liquid sulfur in the presence of a catalyst comprising a basic nitrogen compound.
None of the processes in which hydrogen sulfide is used to convert sulfur dioxide to elemental sulfur are particularly desirable for the reduction of sulfur dioxide recovered from flue gas streams since each requires a source of hydrogen sulfide gas, which is not normally available at the site of flue gas desulfurization plants.
Other processes are known in which the production of sulfur from sulfur dioxide requires that the sulfur dioxide be first reduced to hydrogen sulfide. For example, U.S. Pat. No. 3,932,586 discloses such a process in which sulfur dioxide is absorbed in an aqueous absorption solution including potassium carbonate, and the resulting potassium sulfite or sulfate is treated to regenerate potassium carbonate with the release of hydrogen sulfide, which can then be converted to elemental sulfur by processing in a Claus plant or other sulfur recovery unit. This process has the disadvantage that several intermediate steps are required to produce sulfur from sulfur dioxide.